1. Field of the Invention
This invention relates generally to an oscillator, and more particularly, to a crystal-controlled oscillator operable at a direct UHF frequency.
2. Background Information
The difficulties associated with directly generating pure UHF signals by means of a crystal-controlled oscillator are well known and stem from the design limitations of presently available crystals. In the past, commercially available crystals had a maximum oscillation frequency of approximately 150 MHz, which falls well below the UHF range. Recently, new techniques have been developed by crystal manufacturers to design crystals which are directly operable in the UHF range. These crystals are SC-cut and are capable of operating in a third overtone mode having a range of between 250 and 400 megacycles. Although SC-cut crystals are capable of UHF oscillation, these newly developed crystals have been successfully used only in passive mode filter circuitry. When a UHF-range crystal was operated in a UHF oscillator circuit, the crystal automatically oscillated at a lower, VHF frequency.
Since known SC-cut crystal-controlled oscillators of various designs are incapable of generating a direct UHF signal, other techniques had to be developed for generating monochromatically pure UHF signals. One known technique includes the steps of generating VHF signals utilizing a VHF crystal oscillator typically having a range of between 80 to 100 MHz, and thereafter multiplying the VHF signal to UHF by means of a step recovery frequency multiplier. This technique, however, has a basic limitation in that passing the VHF signal through conventional frequency multiplication circuitry to transform the signal to UHF results in a degradation of the signal phase noise response by the term 20 log n, where n is the frequency multiplication factor. Since many electronic systems (i.e.--radar) are directly affected by oscillator phase noise response, an increase in phase noise degradation results in a corresponding degradation of the electronic system performance.
Therefore, there is a need for a crystal-controlled oscillator of improved design which eliminates the phase noise degradation problems which exist with present crystal-controlled oscillators. The improved oscillator must be capable of generating a direct UHF frequency signal to eliminate the need for presently used frequency multiplication circuitry.